Amplifiers are commonly used to amplify a signal. For example, in radio frequency (RF) or cellular applications, base stations or other infrastructure components employ amplifiers to broadcast signals over greater distances. In practice, amplifiers are often biased to operate in a desired manner for a given application. For example, for communication schemes having relatively high peak-to-average ratios, Doherty amplifier topologies are commonly used to improve efficiency. A Doherty amplifier topology typically includes a pair of amplifiers, a main (or carrier) amplifier and a peaking (or auxiliary) amplifier, where the peaking amplifier is biased to turn on when the input signal increases above a level that would cause the main amplifier to saturate, thereby reducing the impedance at the output of the main amplifier to enable the main amplifier to deliver more current in conjunction with current delivered by the peaking amplifier.
Due to manufacturing variations, the characteristics of a particular amplifier circuit topology will vary across individual devices. Accordingly, biasing amplifiers often involves applying an initial bias voltage, measuring the resulting quiescent current consumed by the amplifier, and adjusting the bias voltage to achieve the desired quiescent current and/or operating mode. In some situations, this is done when the amplifier device is manufactured, such that the bias point of the amplifier device is not adjustable during subsequent operation. Other approaches involve the use of additional external circuitry, which increases the complexity and/or size requirements for the amplifier system, or the introduction of current sensing elements that are electrically in series with the amplifier, reduces the efficiency of the amplifier system.